This question is done under the assumption that the bus has an equal chance at arriving at each minute (i.e bus has equal chance at arriving at 7.30 or 7.31) as it is not stated in the question. First, find the probability the each bus arrive before 7.33am. Assuming both bus arrival timings are independent of each other, where the arrival time of the express bus does not affect the arrival time of the normal bus, the probability that you miss both buses is the product of the probability of the express bus arriving before 7.33am and the normal bus arriving before 7.33am. (Formula: Probability of two independent events occuring=Product of the probabilities of each independent event occuring) Sorry for the complicated language but hope this helps!